1. Field of the Invention
The present invention relates to an exposure apparatus that exposes a substrate to light while the substrate is scanned, and a device manufacturing method employing that exposure apparatus.
2. Description of the Related Art
When manufacturing a device, such as a semiconductor device, a liquid crystal display device, or a thin film magnetic head with a lithography process, an exposure apparatus is used that, via a projection optical system, transfers a pattern of a reticle onto a substrate on which a photosensitive agent has been applied. Higher resolution is sought in exposure apparatuses in accordance with miniaturization and higher density of devices.
The projection optical resolution of the pattern depends on the numerical aperture (NA) of the projection optical system and the exposure wavelength, and is increased by increasing the NA of the projection optical system or shortening the exposure wavelength. The shortening of the exposure wavelength has progressed, for example, in order, from g-ray, to i-ray, to excimer laser light. Shortening of the wavelength of excimer light to 248 nm, 193 nm, and further, to 157 nm, is progressing.
An exposure apparatus may employ, as is well known, a step and repeat system or a step and scan system. An exposure apparatus using a step and scan system is referred to as a scanning exposure apparatus. In particular, with a scanning system, it is possible to match the surface of the substrate to an optimal image plane position while performing scanning exposure, so it is possible to diminish a reduction in exposure precision caused by poor flatness of the substrate.
With a scanning exposure apparatus, before an exposure area (shot region) of the substrate approaches an irradiation area of slit-shaped exposure light (hereinafter, slit light), the surface position of the exposure area can be measured and corrected to the position of the image plane. The surface position is the position in the direction of the optical axis of the projection optical system.
For example, it is possible to measure the surface position using a light oblique incident type surface-position detecting apparatus, or a gap sensor, such as an air microsensor or an electrostatic capacitive sensor. Further, it is possible to arrange or to define a plurality of measurement points in order to measure the tilt of the surface in addition to the height position.
FIGS. 9 and 10 show example arrangements of measurement points. In the example shown in FIG. 9, three measurement points of the surface-position detecting apparatus are arranged in both the front and to the rear in the scanning direction (Y direction) of a slit light 900. In the example shown in FIG. 10, five measurement points of the surface-position detecting apparatus are arranged in both the front and to the rear in the scanning direction (Y direction) of the slit light 900. Measurement points are arranged in both the front and to the rear in the slit direction, so that, with scanning for exposure performed in both the +Y direction and the −Y direction, the surface position of the substrate is measured immediately before exposure in either direction.
Japanese Patent Laid-Open No. 09-045609 discloses a method of measuring the focus and tilt of scanning exposure.
Japanese Patent Laid-Open No. 2004-071851 discloses a method of controlling and driving the focus and tilt using wafer surface information obtained in advance with a focus detecting system provided separately from an exposure apparatus.
The miniaturization trend has been accompanied by the focal depth becoming extremely small, and thus, demands for so-called focus precision, meaning the precision of matching the surface of the substrate to be exposed to an optimal image plane, have become more and more intense.
In particular, it has become clear that in a substrate with an imprecise surface shape, the precision of focus detection in exposure areas becomes a problem. In a numerical example, the required control of flatness of the substrate relative to the focal depth of the exposure apparatus is ordinarily 1/10 to ⅕ of the focal depth, and is 0.04 to 0.08 μm when the focal depth is 0.4 μm. As is shown, for example, in FIG. 7, when the surface position of the substrate is corrected based on the information of measurement points FP1, FP2, and FP3 arranged at equal intervals, information of the surface position of the substrate is not present between the measurement points. Accordingly, an offset amount Δ of defocusing occurs between the actual surface position of the substrate and the surface obtained from the surface position information FP1, FP2, and FP3. Such a defocusing factor is referred to as a focus sampling error.
In order to reduce the focus sampling error, the focus sampling interval should be reduced. Here, for example, the focus sampling error can be determined based on the detection area and the sampling period of the measurement sensor, a sampling period corresponding to the residual vibration mode of the structural body of the exposure apparatus, the control frequency of the control system, or the like. For example, a case is conceivable in which measurement points are arranged at 1 mm intervals in the scanning direction, and light oblique incident type measurement points are arranged at 1 mm intervals in a direction perpendicular to the scanning direction. In this case, the surface information across the entire range of the substrate is obtained as information mapped to a grid of 1 mm intervals in the scanning direction and 1 mm intervals in the direction perpendicular to the scanning direction.
However, at the point of production, products with various chip sizes are produced due to the introduction of a cut-down substrate, a shrunken substrate, and the like, accompanying trends of chip diversification and miniaturization. Accordingly, with the arrangement of measurement points depending on the performance and configuration of the apparatus as described above, the positional relationship of a shot and measurement points changes each time the shot is crossed. As a result, local defocusing occurs at a portion near the edge of the shot, particularly, in a case in which the distance increases between a measurement point and the position where exposure starts and/or the position where exposure ends. A shot region can be configured to include one or a plurality of chip areas.